Fragile X syndrome, a common form of inherited mental retardation, is caused by the loss of the fragile X mental retardation protein (FMRP). FMRP is a selective RNA-binding protein that forms a messenger ribonucleoprotein (mRNP) complex associating with polyribosomes. Evidence suggests that FMRP is involved in local regulation of protein synthesis at synapses. The loss of FMRP leads to abnormal translation of selective mRNAs, delayed maturation of dendritic spines, and abnormal behavioral phenotypes. However, mechanism by which FMRP regulates the translation of its mRNA ligands remains unclear. Since Drosophila model allows powerful genetic and molecular manipulations, in the last several years, fruit fly has been increasingly used to study fragile X syndrome. Phenotypic analyses have demonstrated an array of neuronal and behavioral defects similar to the phenotypes reported in mouse models as well as in human patients. The long-term goal of this proposal is to delineate the molecular pathogenesis of fragile X syndrome using Drosophila as a model system. MicroRNAs (miRNAs) are a new class of noncoding RNAs that are believed to control translation of specific target mRNAs by pairing with the mRNA in an antisense manner. Members of the PIWI/PAZ-domain protein (Argonaute) family facilitate processing and downstream functions of miRNAs. The recent studies from our group and other laboratories have demonstrated biochemical and genetic interaction between FMRP and the components of the miRNA pathway, including Dicer and Argonaute proteins, suggesting that FMRP could potentially utilize the miRNA pathway to regulate the translation of its mRNA ligands, and modulate cellular and behavioral phenotypes. Here we propose to further decipher the functional importance of the interaction between FMRP and the miRNA pathway using the available Drosophila Argonaute mutants (dAGO1 and dAGO2) along with fragile X models. Three specific aims are proposed: 1) To test the hypothesis that dFmr1 protein interacts with specific miRNAs and determine the role of dFmr1 protein (dFmrp) in miRNA-mediated translational regulation. 2) To determine the domains required for the interaction between dFmrp and Argonaute proteins, and to test the hypothesis that the level of Argonaute proteins modulates c/Fmrp-mediated neuronal plasticity and behavioral activities in vivo. 3) To identify and characterize the genetic modifiers of dAGO1.